1. Field of the Invention
The present invention relates to a switch and a switching method for performing a switching operation for each cell and a process for accommodating various types of data accommodated within a different network in a communications network where data is partitioned into cells being fixed-length packets, which are transferred.
2. Description of the Related Art
An ATM (Asynchronous Transfer Mode) communications method is a method for partitioning data having various speeds into fixed-length packets referred to as cells, and for transferring the cells. This method can process various types of data having diversified traffic characteristics in a unified manner in a communications network. Therefore, a communications network adopting the ATM communication method (ATM network) has been built as the infrastructure of multimedia communications.
ATM switches are arranged in such an ATM network, and relay ATM connections between users by performing a switching operation for each cell. For example, the ATM switch disclosed by the Japanese Laid-open Patent Application No. 7-307745 has the configuration shown in FIG. 1A.
An ATM switch 1 shown in FIG. 1A is mainly composed of a switch unit 2 and a plurality of line units 3 (subscriber line units or trunk line units). One or a plurality of subscriber terminals 5 are connected to each of the line units 3 directly or via a transmitting device 4, and one or a plurality of connections (ATM connections) from one or a plurality of subscribers are accommodated by each of the line units 3.
As described above, the ATM switch 1 can accommodate a plurality of connections on a single input port (line unit). To which connection a cell belongs is identified with a VPI (Virtual Path Identifier) and a VCI (Virtual Channel Identifier), which are included in the header of the cell. A connection identified only with the VPI is referred to as a VP connection, while a connection identified with the VPI and the VCI is referred to as a VC connection.
When a connection is relayed by the ATM switch 1, a VPI/VCI conversion unit 6 arranged within the line unit 3 converts the VPI/VCI of an input cell into those corresponding to an output line, and attaches tag information (TAG) to the cell by referencing a VPI/VCI conversion table 7, as shown in FIG. 1B. The TAG is an internal identifier used for selecting a route within the switch unit 2.
The switch unit 2 outputs a cell to a desired route by using the TAG attached to the cell. With these operations, the ATM switch 1 can switch each of input cells to a desired line.
By the way, an ATM network can accommodate subscriber terminals other than ATM terminals. As such terminals, for example, terminals connected to an existing STM (Synchronous Transfer Mode) communications network which mainly handles telephony services can be cited.
If an STM subscriber terminal is accommodated by an ATM network, an STM-ATM converter called an IWF (InterWorking Facility) or a CLAD (Cell Assembly and Disassembly) is installed between the STM and ATM networks. The STM-ATM converter converts STM data into an ATM cell, and transfers the STM data within the ATM network.
FIG. 1C is a block diagram showing the network configuration where an ATM network accommodates an existing STM network. In this figure, an ATM network 11 includes a plurality of ATM switches 1, to each of which an ATM subscriber terminal 5 is connected. An STM network 12 includes a plurality of STM switches 13, to each of which an STM subscriber terminal 14 is connected. IWFs 15 are arranged between the ATM switches 1 and the STM switches 13.
For example, data transmitted from the STM subscriber terminal 14 in the lower left of this figure is input to the ATM network 11 via the STM switch 13 and the IWF 15, and is transferred over a trunk line within the ATM network 11. Then, the data reaches the STM subscriber terminal 14 in the lower right via the IWF 15 and the STM switch 13. Or, the data may sometimes reach the ATM subscriber terminal 5 in the upper right, which is connected to the ATM network 11.
In this case, the following two methods can be considered as a data mapping method used when STM data is converted into ATM cells (hereinafter referred to as cell processing) or ATM cells are converted into STM data (hereinafter referred to as decell processing) within the IWF 15.
(a) A method using an AAL (ATM Adaptation Layer) type 1 (ITU-T (International Telecommunications Union-Telecommunications) Recommendation I.36.3.1)
(b) A method using an AAL type 2 (ITU-T Recommendation I.363.2)
The AAL type 1 is also referred to as an AAL1. This is a cell processing method for transmitting STM data of 47 bytes, each of which has a 125-xcexcs speed, is transmitted as one cell. This method is suitable for the case where the cell processing is performed for data at a fixed-rate speed. Additionally, an AAL type 2 is also referred to as an AAL2. This is a cell processing method for mapping data onto short packets of a variable length, which are referred to as short cells, and for multiplexing a plurality of short cells into a single ATM cell. This method is suitable for the case where the cell processing is performed for data of variable and low-speed data.
When the IWF 15 performs the cell processing for STM data, it reduces the amount of the data by performing voice encoding (including silence suppression as occasion demands), and performs the cell processing for the encoded data with the AAL type 2. The silence suppression means that data is not transferred if it is in an unvoiced state.
Such cell processing allows the bandwidth compression of data in an ATM network. Note that, however, the bandwidth compression is implemented based on the assumption that the IWF 15 or the ATM subscriber terminal 5, which is the destination of a connection, supports the AAL type 2. For a non-voice signal for which the bandwidth compression cannot be performed, a transfer with the AAL type 1 is more useful than that with the AAL type 2 in terms of a bandwidth.
The above described conventional communications methods, however, have the following problems.
Normally, if a transmission delay exceeds 25 ms on either of calling and called sides at the time of a voice signal transfer, an echo caused by this delay cannot be ignored and the echo must be compensated for by an echo cancellor. When STM data is transferred within an ATM network, a delay specific to the ATM network and a delay which accompanies the cell or decell processing performed by the IWF occur. Therefore, a delay time may be larger than that in an existing STM network.
FIG. 1D is a schematic diagram showing a delay which accompanies the cell/decell processing performed by the IWF with the use of the AAL type 1. When the IWF generates a single ATM cell 22 by performing the cell processing for 47-byte STM data 21, a cell processing delay depending on each of the bytes occurs. By way of example, a cell processing delay of approximately 6 ms (125 xcexcsxc3x9747) occurs for the leftmost 1-byte data 21xe2x80x2, while a cell processing delay is recognized to be xe2x80x9c0xe2x80x9d for the rightmost 1-byte data 21xe2x80x3.
When the ATM cell 22 is transferred over an ATM network, a transmission line delay and a delay variation absorption time xcfx84 within the ATM network are added. Normally, cells are buffered in order to prevent a loss caused by a cell conflict within the ATM network. The delay variation absorption time xcfx84 is required to absorb the delay variations of cells caused by the buffering.
Furthermore, when the STM data 21 is regenerated by performing the decell processing for the ATM cell 22 by the IWF at a transfer destination, a decell processing delay occurs depending on each of the bytes. By way of example, a decell processing delay is recognized to be xe2x80x9c0xe2x80x9d for the data 21xe2x80x2, while a decell processing delay of approximately 6 ms occurs for the data 21xe2x80x3.
Accordingly, a delay accompanying the cell/decell processing can be estimated to be approximately 6 ms, and the total delay time can be calculated by adding the transmission line delay and the delay variation absorption time xcfx84 to the cell/decell processing delay. Occurrences of the cell/decell processing delay and the delay variation absorption time mean that an area in which an echo must be compensated for increases for an STM subscriber receiving an existing analog telephone service.
If the capability provided by an IWF is the cell/decell processing of the AAL type 1, a CLAD of the AAL type 1 must be arranged within the IWF. Also an echo cancellor must be arranged to compensate for the above described echo caused by a cell delay.
FIG. 1E is a block diagram showing the configuration of a hypothetical IWF equipped with the above described devices. An IWF 31 shown in FIG. 1E accommodates xe2x80x9cmxe2x80x9d channels, and comprises echo cancellors 33 the number of which is equal to the number of channels, and an AAL1 multiplexing CLAD 32. The AAL1, multiplexing CLAD 32 comprises AAL1-CLADs 34 the number of which is equal to the number of channels, and an ATM demultiplexing unit 35. Here, the AAL1-CLADs 34 are arranged for the respective channels. However, a multiplexing CLAD for processing a plurality of channels altogether was developed (for example, the Japanese Laid-open Patent Application No. 5-37548).
If the capability provided by the IWF is cell/decell processing of the AAL type 2, a CLAD of the AAL type 2 must be arranged within the IWF. Also an echo cancellor is required due to a reason similar to that for the AAL1 type.
FIG. 1F is a block diagram showing the configuration of a hypothetical IWF equipped with these devices. An IWF 41 shown in this figure accommodates m channels, and comprises echo cancellors 42 and voice encoders 43, the numbers of which are equal to the number of channels, and an AAL2-CLAD 44.
Normally, signals of a plurality of channels are multiplexed at an input/output point on an STM network side of an IWF in many cases. Each of the channels accommodates various types of data such as an analog voice, bearer data, etc. The bearer data indicates, for example, data transferred by a bearer service with the use of a dedicated line, and is not limited to voice data.
As described above, an optimum AAL type differs depending on the type of data. Therefore, it is desirable to change the AAL type depending on the type of data in order to make efficient use of an ATM network.
For example, if a connection destination accommodates the AAL type 2 at the time of an analog voice transfer, it is desirable to perform the cell processing of the AAL type 2. If the connection destination does not accommodate the AAL type 2, it is desirable to perform the cell processing of the AAL type 1. When non-voice data is transferred, it is desirable to perform the cell processing of the AAL type 1. For an analog voice, an echo cancellor must be installed depending on an amount of an end-to-end transmission delay time.
However, if the IWF is designed to provide both of the capabilities of the AAL types 1 and 2, and to provide the capability of an echo cancellor depending on a connection, the configurations shown in FIGS. 1E and 1F must be combined. In this case, echo cancellors, voice encoders, etc. must be arranged for all of channels accommodated by the IWF, so that an amount of equipment becomes very large.
Especially, digital signal processing using a DSP (Digital Signal Processor), etc. must be performed within each echo cancellor and a voice encoder. Accordingly, their hardware sizes are significantly large. If echo cancellors and voice encoders for all the channels are arranged, the total size of the IWF becomes extraordinary.
Furthermore, if encoding algorithms of voice encoders are different depending on IWFs such as an IWF at a connection destination, etc., a plurality of voice encoders or a voice encoder accommodating a plurality of encoding algorithms must be arranged for each channel.
An object of the present invention is to provide a switch and a switching method for efficiently accommodating various types of data accommodated within a different communications network such as an STM network, etc., and for reducing an amount of equipment of interworking facilities between networks in a communications system such as an ATM network, etc., which partitions data into fixed-length packets and transfers the packets.
In a first aspect of the present invention, a switch comprises a switch unit and a trunk unit, and switches communication data, which is partitioned into cells being fixed-length packets, for each of the cells. The switch unit determines an output route for each input cell. The trunk unit performs voice processing for an input cell if the cell belongs to a particular voice connection.
The switch unit performs a switching operation, for example, based on the VPI/VCI being the connection identification information of an input cell, and determines an output route according to a connection. If the input cell is identified to belong to a particular voice connection at this time, the switch unit outputs this cell to the trunk unit.
The trunk unit performs one of an echo cancellor process for removing an echo of a far end signal superposed on a near end signal, an encoding process for converting voice data of an input cell into a particular voice code, a decoding process for converting a voice code of an input cell into voice data, a multiplexing process for multiplexing data of an input cell into an AAL type 2 cell, a demultiplexing process for demultiplexing data from an AAL type 2 cell, etc. and again inputs the cell to the switch unit.
In a second aspect of the present invention, a trunk is arranged for a system which switches communication data, which is partitioned into cells being fixed-length packets, for each of the cells, and comprises a voice processing unit and a conversion unit. The voice processing unit performs voice processing for an input cell if the cell belongs to a particular voice connection. The conversion unit converts the connection identification information of that cell.
In a third aspect of the present invention, an switch comprises a switch unit and a trunk unit, and switches communication data, which is partitioned into cells being fixed-length packets, for each of the cells. The switch unit determines an output route for each input cell. The trunk unit processes data of an input cell if the cell belongs to a particular connection.